1. Field of the Invention
The present invention relates to a method of accelerated preparation of unsaturated polyesters based on maleic acid/anhydride.
2. Discussion of Related Art
It is known that the synthesis of unsaturated polyester resins, i.e. a polyesterification, usually occurs in the absence of a catalyst, since the protons from the COOH groups of dicarboxylic acids serve as catalyst (P. H. Selden: Glasfaserverstarkte Kunststoffe, Springer Verlag, Berlin-Heidelberg-New York, 1967, p. 16).
The most widely used acid component of unsaturated polyesters is maleic acid/anhydride, which is used as a strong acid, and in most cases, provides appropriate proton concentrations to the catalysis (I. Vancso-Szmercsanyi, E. Makay-Body: J. Polym. Sci., Pt. C, 16, 3709/1968/). During esterification, however, the concentration of the COOH groups decreases, and consequently, the reaction proceeds at a decreasing rate due to the exhausting of the amount of catalytic protons (P. J. Flory: J. Amer. Chem. Soc., 61, 3334/1939/).
It has now been discovered experimentally that the rate of polyesterification can markedly be increased by the addition of a small porton of maleic acid/anhydride to the reaction mixture during an advanced stage of the reaction.
It has been found that the rate of reaction may be increased more than twofolds, i.e. the manufacturing cycle can be reduced to about one half. In addition, the proceeding of the reaction is practically linear in time and without any slow-up. The significance of these observations is implied in relation with the experimetal experience which was provided by the method of the present invention permits a simple procedure for the preparation of high-molecular-weight unsaturated polyesters.
The reduction in the reaction period involves favorable consequences to quality improvement. E.g. it is known that synthesis of maleic acid-based polyesters are accompanied by undesirable side-reactions. The most considerable one is a partial addition of alcoholic OH groups to some of the unsaturated bonds (Z. Ordelt: Makromol. Chem., 63, 153/1963/). As a consequence, unsaturation of polyester chains may decrease even by as much as 10 percent, leading to a reduction in cross-link density of the copolymerization-cured resin. Another deleterious effect of the said side-reaction is a chain branching. In the procedure provided by the present invention, the unfavorable side-reaction is suppressed.
Industrial unsaturated polyesters are mixed esters synthesized from several types of carboxylic acids and alcohols. The most current unsaturated component is maleic acid/anhydride which is accompanied by saturated dicarboxylic acids, such as phthalic, sebacic, succinic acids, etc. In a single-step esterification procedure, these components are reacted simultaneously.